1. Field of the Invention.
The present invention relates to control circuits for pump motors. More specifically, the field of the invention is that of liquid level control circuits which automatically maintain the liquid level within a predetermined range.
2. Description of the Related Art.
In sump and water tanks, for example, the liquid level should be maintained within a predetermined range for proper functioning of the tank. Many prior art devices automatically control the liquid level within the tank by activating a pump when the liquid rises above a first predetermined level and deactivating the pump when the liquid level falls below a second predetermined level. Some of the prior art devices use mechanical or moving parts such as mechanical switches operated by rubber diaphragms, springs, rods, floats, or balls, all of which may tend to wear out or malfunction over time.
Other prior art devices use electrical or optical probes positioned within the tank to determine the liquid level and control the pump accordingly. For example, self-heating thermistors or conductivity probes may be used. However, such prior art systems using probes may be sensitive to humidity, moisture, changing temperatures, and varying voltage levels in the sensing circuit, all of which may produce erroneous results and subject the probes to wear. Also, contamination of the probes may adversely effect their performance. The probes and their associated circuitry may be adjusted to improve performance, but making the adjustments may be inconvenient and expensive.
Employing capacitive sensors for liquid level control provides advantages including the prevention of triggering from transient water imbalances, such as splashes or waves, by precisely defining the required charging time of the capacitive sensors. However, the charging time may vary over operating temperatures in each unit, and similar units may vary in charging time because of variations in the electrical components of the control. Such variations may cause some controls to be activated falsely. In order to prevent any such occurrences, a desired feature of a liquid level control is to minimize variations in charging time, and therefore the time needed for actuation.
The charging time required for capacitive sensors is at least in part determined by the size of the resistors in the charging circuit. For applications which charge the capacitive sensors solely on the basis of the alternating current cycle (60 Hz), the resistances required are large in order to keep the capacitive sensors from completely charging every cycle. However, one problem with large resistances is that they are susceptible to receipt of radio frequency (RF) energy due to the antenna effect. The antenna effect or strays from the AC source may cause unexpected charging of capacitive sensors.
What is needed is a liquid level control which minimizes variations in actuation time.
Another need is for a liquid level control which reliably operates without the need for adjustment.
Also needed is a liquid level control which minimizes operating problems associated with contamination and mechanical wear.
A further need exists for a liquid level control which minimizes inaccuracies associated with varying temperatures and high resistances.